The invention relates to acoustic devices of the kind comprising a sound radiating member relying on bending wave action and resulting surface vibration to produce acoustic output.
It is known from U.S. Pat. No. 3,247,925 of WARNAKA to suggest a low frequency loudspeaker consisting of an extremely rigid resonant panel, the peripheral edges of which are bolted or cemented to a rigid frame, which frame supports a conventional voice coil transducer which imparts bending wave energy to the centre of the panel. This low frequency loudspeaker device is said to operate entirely above wave coincidence frequency.
It is also known from U.S. Pat. No. 3,596,733 of BERTAGNI to propose a loudspeaker having a diaphragm formed by an expanded polystyrene plate-like member having a pre-tensioned front face and a rear face of or including an irregular shape.
Revelatory teaching concerning bending wave action acoustic devices, conveniently considered as generally of resonant panel type, is given in International Patent Application W097/09842, including as to improvement or optimisation of acoustic performance according to panel parameters including geometry and bending stiffness; particularly including operation usefully at and below coincidence frequency. Geometrical parameters of interest include proportions or aspect ratios of panels as such, including for use as passive acoustic devices. Parameters of bending stiffness(es) can usefully interact with geometric parameters, including anisotropy thereof, say as to different bending stiffnesses of or resolvable to substantial constancy along axes of geometric shapes involved for viable variation of proportions of such shapes. Preferential in-board locations for transducers of active acoustic devices usefully have proportional defining coordinates. Other areal distributions of bending stiffness can usefully contribute to affording other useful locations for transducers, for example substantially at geometric centres and/or at centres of mass, see International Patent Application WO98/00621 including for combining aforesaid bending wave action with further acoustically relevant pistonic action. Acoustic operation is described and claimed in at least W097/09842 for both of whole panels and only parts thereof being acoustically active.
On an intuitive basis, our specific analysis and design methodology to date for such resonant mode bending wave action acoustic devices has been mainly concentrated upon whole panels where edges are wholly or substantially free to vibrate when in acoustically relevant desired bending wave action, including where subject to light edge damping. This invention arises from surprising results of counter-intuitive further consideration, research and experimentation.
Certain under-lying requirements continue to apply and be of profound technological/inventive significance, specifically for an acoustic device member extending transversely of its thickness and capable of sustaining bending waves through its consequentially acoustically active area, i.e. basic requirement for what is herein called a resonant acoustic member or panel; and for parameters such as geometric and for bending stiffness to be of values consonant with resulting distribution of natural bending wave vibration of said member that is effective in or beneficial to achieving desired or acceptable acoustic operation of the device over a frequency range of interest, i.e. further requirement for a resonant acoustic member or panel hereof. Specific embodiments of this invention additionally provide for means affording substantial restraint of bending wave vibration typically at edge, periphery or other boundary of such member or panel or acoustically active area thereof, and further typically to be at least capable of operating at least partly below coincidence frequency. The wording xe2x80x98substantial restraintxe2x80x99 as used herein intentionally involves greater constraining of at least part(s) of edge(s) of the member than specifically disclosed in W097/09842, preferably as to both of edge extent(s) and effective loading, grip or effective grounding effect.
There are two views, effects or inventive aspects that it is seen as useful to consider relative to such substantial edge/areally bounding/peripheral restraint.
One is that limitation/reduction of available bending wave vibrational edge/peripheral/boundary movement of the member (compared with specific disclosure of WO97/09842) can produce useful compounding of achieved acoustic output from vibrational bending wave energy back in the acoustically active area. The other is that the acoustically relevant and effective natural modes of resonant bending wave action will be different (compared with specific disclosure of WO97/09842) by reason of limiting/suppressing bending wave vibration movement at edge(s)/periphery/boundary of the member, thus effectively reducing/eliminating contributions(s) from lowest resonant mode(s) that would be active if edge(s)/periphery/boundary of the acoustically effective area of the member were as free to have bending wave distribution as specifically disclosed in WO97/09842; and reduction/substantial suppression of resonant modes involving twisting.
Resulting nominally less populous or less rich content of acoustically active/relevant resonant bending wave modes can be exemplified for simplified analogy and analysis based on equivalent simple beams with account taken of interactions, in terms of involving resonant plate modes that relative to each beam start at resonant mode frequency f1 rather than f0, and further xe2x80x98losingxe2x80x99 combinational modes involving f0 frequencies, but with interesting and useful effects available with respect to even-ness of spacings of directly and combinationally related natural resonant modes involving f1 frequencies.
Ramifications are extensive and can be advantageous, including attainability of improved acoustic efficiency of energy conversion and/or often very usefully increased extents of candidate sub-areas for viable/optimal transducer location(s), at least as identified by mechanical impedance analysis as taught in co-pending International patent application PCT/GB99/00404; and/or typically much greater range of viability of areal shapes/proportions of said members as exemplified for isotropic bending stiffness, even at about 1:1 through to about 1:3 and more for aspect ratio(s); and/or viability of acoustic performance for panel member materials of lower intrinsic bending stiffness at least as effectively stiffened overall by contribution from edge(s)/peripheral/boundary restraint hereof; and/or capabilities in relation to high power input transducer means for loudspeaker embodiments, all including where such restraint can afford substantial loading whether on an inertial grounding basis or as is further practical by actual fixing in a more rigid/massive carrier or other heavy loading manner.
It is a significant advantage of this invention that novel and useful resonant panel acoustic devices are provided, including active acoustic devices as loudspeakers, with significant facilitation of manufacture as robust readily-mounted panel-type devices, particularly enhanced relative to acoustic devices specifically illustrated and described in International Patent Application W097/09842.
According to one aspect of the present invention there is provided an acoustic device relying on bending wave action and capable of operating below coincidence, comprising a member affording said acoustic operation by reason of beneficial distribution of resonant modes of bending wave action therein, wherein the member has its acoustically active area at least partly bounded by means having a substantially restraining nature in relation to bending wave vibration.
According to another aspect of the present invention there is provided an active acoustic device comprising a member relying on bending wave action with beneficial distribution of resonant modes thereof and beneficial location of bending wave transducer means, wherein the member has its acoustically active area at least partly bounded by means having a substantially restraining nature in relation to bending wave vibration, and its transducer means location determined with reference to and taking account of such bounding means.
The entire periphery of an acoustic member hereof may be substantially restrained, or clamped; or only part(s) less than all of periphery of the member, e.g. a rectangular panel, may be restrained or clamped at one or more up to all of its side edges. This can be useful as a flag-like mounting affording said substantial restraint at one side with the acoustically active area protruding therefrom, or as mounting at two sides that may be parallel and afford said substantial restraint with the acoustically active area between those mounting and restraining sides; and can facilitate the manufacture of up to fully sealed or only highly selectively vented diaphragm loudspeakers, e.g. mid/high frequency devices. A fully or near-fully sealed diaphragm enables the making of a so-called infinite baffle loudspeaker to contain/control rear acoustic radiation which might otherwise be detrimental at mid to low frequencies.
Full substantially restraining or clamping frames also enable design of the loudspeaker assembly to be more predictable in mechanical terms, along with facilitating making a loudspeaker assembly which is relatively robust in construction (compared to a resonant panel loudspeaker in which the panel edges are substantially free or are suspended in an only lightly damping resilient manner).
Substantial restraint or clamping of peripheral portion(s) or edge(s) of the acoustic member may be achieved in any desired manner, e.g. by intimately fixing the edge(s) to a strong frame or the like by means of an adhesive, or by mechanical means say involving clamping the edge(s) between frame members. The desired edge restraint/clamping hereof may also be achieved by moulding techniques (such as injection moulding of plastics materials) by forming the edges of the member with integral or integrated thickened surround portions of sufficient rigidity to terminate edge movement of the acoustic member. Co-moulding of the acoustic member and thickened edge provision may be appropriate. Such moulding techniques may be particularly suitable where the acoustic member is formed as a monolith and may be readily achievable in economic manner.
Substantial restraint or clamping may also be used to define one acoustic member within another larger acoustic member. Thus a large acoustic panel intended for mid/low frequency operation may be moulded to include a smaller acoustic panel intended for high frequency operation and defined by medial stiffening ribs.
Substantial restraint or clamping action can be designed to present a mechanical termination impedance designed to control the reverberation time within the acoustic member as an aid to control of the frequency response of the member, perhaps especially at lower frequencies.
Proportions of suitable resonant panel members may be as or substantially different from specific teaching of WO97/09842 regarding variations on particular shapes. For example, substantially rectangular resonant panel members of substantially isotropic bending stiffness could be of aspect ratios below 1:1.5 then generally inclusive of prior teaching for substantially free edge panel members but not limited thereto as will be specifically described later herein, or greater than 1:1.5 as will also be specifically described later herein. Variations for anisotropy/complex distribution of bending stiffness(es) is envisaged as above.
The bounding means may be at least partially about and definitive of said acoustically active area and/or about peripheral edge(s) of a panel-form member to be wholly acoustically active, typically to extent of up to 25% or more of full area boundary/peripheral edge extent, often the whole thereof.
Resonant panel members are generally self-supporting and would not require pre-tensioning for mechanical stability, particularly for types typical of free edge or simple edge supported use.
For clamped panel member there is a ten-fold or thereabouts increase in first bending frequency due to the natural stiffening of the panel member when clamped. It is logical and sensible to substantially reduce the bending stiffness property to reduce the first modal frequency and before the lower frequency range. It is envisaged that the stiffness of panel member in such cases may be as low as 0.001 Nm and the area density as small as 25 g/m2.
From one viewpoint these ends of range values describe a panel member which for mechanical stability and the function for drive means support may benefit from the application of tensioning forces. These may be applied uniformly or differentially, i.e. in different directions and/or at different tensions, with respect to the effective geometry of the member.
At the limit the tensioned panel exhibits a high proportion of the properties of a tensioned film supporting bending waves and with predominantly second order or non-dispersive wave action (velocity constant with frequency). For such a xe2x80x98panelxe2x80x99 member the resonant distribution may be optimised for desired acoustic behaviour by control of tensioning and boundary geometry in broad agreement with distributed mode teaching, see WO97/09842. Likewise a preferred modal distribution may be further augmented into action as a transducer via preferred/optimised placement of the exciter/sensor.
Depending on the degree of tensioning and with increasing density and more particularly bending stiffness, there will be a range where second order bending wave action is superimposed and augmented by fourth order, dispersive bending action-due to stiffness. Optimisations of the two may be derived by calculation and/or experiment to provide the best results in a given application.
Smaller wide-bandwidth acoustic panels with edge clamping are the envisaged field of design.